There are few data for hormonal levels and testis structure and function during postnatal development in rats neonatally-treated with monosodium L-glutamate (MSG). In our study, newborn male pups were i.p. injected with MSG (4 mg/g BW) every 2 days up to 10d of age, and investigated at prepubertal and adult ages. Plasma levels of leptin, LH, FSH, prolactin, T, corticosterone and free thyroxin (FT₄) were measured. MSG-rats displayed elevated circulating levels of corticosterone and hyperadiposity/hyperleptinemia, regardless of the age examined; conversely, circulating prolactin levels were not affected. Moreover, prepubertal MSG-rats revealed a significant (P < 0.05) reduction in testis weight and the number of Sertoli (SC) and Leydig (LC) cells per testis. Leptin plasma levels were several-fold higher (2.41 vs. 8.07; P < 0.05) in prepubertal MSG rats, and these animals displayed plasma LH, FSH, T and FT₄ levels significantly (P < 0.05) decreased. Taken together, these data indicate that testis development, as well as SC and LC proliferation, were disturbed in prepubertal MSG-rats. Adult MSG-rats also displayed significantly higher leptin plasma levels (7.26 vs. 27.04; P < 0.05) and lower (P < 0.05) LH and FSH plasma levels. However, T and FT₄ plasma levels were normal, and no apparent alterations were observed in testis structure of MSG-rats. Only the number of SC per testis was significantly (P < 0.05) reduced in the adult MSG-rats. In conclusion, although early installed hyperadipose/hyperleptinemia phenotype was probably responsible for the reproductive axis damages in MSG animals, it remains to be investigated whether this condition is the main factor for HPG axis dysfunction in MSG-rats.